Conventional communication systems typically operate on the premise that concentrating communication energy to a narrow bandwidth overcomes conflicts with other communication systems sharing the same frequency band by avoiding the specific frequencies that the other communication systems are using. Spread spectrum communication systems have been developed to provide enhanced communications capabilities by spreading the communication energy over a relatively wide bandwidth, rather than concentrating the communication energy in a relatively narrow bandwidth.
A spread spectrum communication system reduces conflicts with conventional communication systems by having a very low energy density at any particular frequency. Conflicts are reduced because a conventional narrowband receiver tuned to a particular frequency will preferentially respond to a narrowband signal at that frequency (especially when using a frequency modulation technique, due to the "capture efect"), while a spread spectrum communication system receiver that is adapted to receive signals spread over a relatively wide bandwidth (in a manner to be described below) preferentially responds to spread spectrum communication signals.
An important factor in the operation of a spread spectrum communication system is the manner in which the communication energy is distributed over the available (relatively wide) frequency bandwidth. Spread spectrum communication systems use conventional carrier modulation techniques to apply the information to be transmitted to the carrier frequency. The information can be in analog or digital form. If the information is supplied in digital form, its rate is known as the data rate. After the first modulation, however, a spreading code is then applied to the information-modulated carrier [a "chipping rate") to spread the communication energy over the wide bandwidth available to the spread spectrum communication system.
There are essentially four ways in which the carrier is spread out. These are frequency hopping, direct sequence, chirp, and time hopping. In the frequency hopping technique, a pseudo-random list of distinct channels is developed and the information-modulated carrier is made to hop from one channel to the next according to the list. In the direct sequence approach, a pseudo-random sequence is mixed with the carrier in order to change the phase or frequency of the information-modulated carrier at a very fast rate. If the phase shifting (phase shift-keying) is accomplished by a balanced mixer, and the pseudo-random sequence is a binary sequence, the phase shifting is typically produced by shifting the information-modulated carrier between 0 and 180 degrees (binary phase-shift keying--BPSK). Quadrature phase-shift keying (QPSK), in which the information-modulated carrier is shifted among four different phases is another common technique of direct sequence spread spectrum communications. Other forms of modulation are quadrature amplitude modulation (QAM), frequency shift-keying (FSK), and multiple phase shift-keying (MPSK). Chirp spread spectrum causes the frequency of the information-modulated carrier to be swept along predetermined frequency ranges. Time hopping operates by causing the information-modulated carrier to be keyed on and off at a very low duty cycle, in accordance with a pseudo-random binary sequence. The spread of the signal is established by the keying speed.
It is also possible to have hybrid spread spectrum communication systems, in which desirable features of two or more of the four most common spreading techniques briefly described above can be combined to suit particular circumstances. Further details of spread spectrum communication systems are given in "Spread Spectrum Systems," by R. C. Dixon, John Wiley and Sons, New York, 1984; "Spread Spectrum Techniques," by R. C. Dixon, IEEE Press, Piscataway, New Jersey; and "Coherent Spread Spectrum Systems," by Holmes, Wiley Interscience, New York, 1982. These references are hereby incorporated by reference.
One important aspect of many common spread spectrum communication systems is the development of the pseudo-random (PN) sequence which is used to spread the information-modulated carrier signal. Typically, the PN sequence chosen in a particular spread spectrum communication system has desirable statistical properties which allow the signals transmitted by the particular system to be suitably distinguished from the signals transmitted by other communication systems. In addition, choices of PN sequences can affect speed of acquiring synchronization of the received signals, which relate to the efficiency of initiating interpretation of the received signals. Common PN sequences are m-sequences, Thue-Moore sequences, and Gold sequences. Their choice and implementation are discussed in "Shift Register Sequences," by S. Golomb, Aegean Park Press, Laguna Hills, California, 1982. Excellent general references to spread spectrum communication systems are "Spread-Spectrum Applications in Amateur Radio," by W. E. Sabin, QST, ARRL, Newington, Connecticut, July, 1983; "Spread Spectrum Theory and Projects," 1993 ARRL Handbook, ARRL, Newington, Connecticut, 1993; "The Spread Spectrum Concept," by R. A. Scholtz, IEEE Trans. on Comm., IEEE, Vol. COM-25, No. 8, August 1977, Piscataway, New Jersey; and "The Origins of Spread Spectrum Communications," IEEE Trans. Comm., May 1982, pp. 822-854, Piscataway, New Jersey. Each of these five further references is also hereby incorporated by reference.
In commercial communications systems, spread spectrum techniques inhibit the casual listener from deciphering transmitted digital data. However, security is not normally a major goal in implementing a digital data spread spectrum system, since the data can easily be encrypted in software before it modulates the carrier signal.
While spread spectrum systems distribute communication energy over a wider bandwidth than narrowband communication systems and consequently benefit an independent co-frequency receiver, they also impair an independent adjacent frequency user. The amount of impairment is dependent upon the relative strengths of the desired and undesired signals at the receiver as well as various spread spectrum parameters that describe each of the channels used in a particular spread spectrum communication system. The relative powers of the desired and undesired signals are affected by transmitter and receiver antenna patterns and the relative positions of the desired and undesired transmitters (also known as the near/far problem).
In addition to reducing the effects of fading on frequency modulation signals due to multipath interference, a spread spectrum communication system reduces intersymbol interference due to ghosting (a particularly important consideration at data rates greater than about 1 megabit per second).
The Federal Communications Commission has allowed unlicensed operation of wireless communication systems in the frequency ranges of 902-928 MHz, 2400-2483.5 MHz, 5725-5850 MHz and 24.0-24.25 GHz If such a system is a narrowband system, it is limited to a maximum transmitted power of 0.75 milliwatt (mW) (although if the output signal is adequately scrambled, above 1000 MHz, the maximum transmitted power can be up to 100 mW). However, if the system is a spread spectrum system, its output power is limited to a much greater maximum of 1 watt (W).
What are unknown in the prior art are methods and apparatus for modifying the transmitter and receiver modulation as well as the transmitted power parameters to achieve the desired data rate of a spread spectrum communication system while minimizing interference to other operating wireless communication systems.